Cable TV systems, of which there are over 10,000 in the United States, serve customers on a paid subscription basis. A cable system operator customarily restricts access to some of the TV channels being sent to each customer depending on which channels or programs the customer pays to receive. Access by a customer to the various channels available in a cable TV system is controlled by means of respective subscriber terminal addresses and access codes sent to each subscriber terminal from a central office over the cable along with the television signals. Each subscriber location is provided by the cable operator with a special subscription TV receiver (subscriber terminal) having a unique address that is normally leased to the customer on a monthly basis. The receiver is owned and controlled by the cable TV operator. To prevent non-authorized access to one or more of the many TV channels being simultaneously transmitted over the cable, the cable TV operator typically uses not only subscriber terminal address and channel access codes, but also employs some kind of proprietary coding system for scrambling and descrambling the actual television pictures of some of the channels. Typically, the picture of a TV channel is individually scrambled at a central office and then individually descrambled by circuitry in the receiver at each subscriber location. Each receiver receives from the central office coded data signals which instruct the respective receivers to descramble (or not) those TV channels for which access is authorized (or not). Thus TV channel access on a subscriber-by-subscriber subscriber basis is easily controlled electronically by the cable TV operator from the central office.
A very effective and widely used way of scrambling a TV picture is to suppress the synchronizing signals which are a normal part of commercial TV broadcasts. In the United States television signals broadcast over their respective channels have synchronizing signals and line video signals in accordance with the standards set by the National Television Standards Committee (NTSC). There are 525 horizontal lines in each frame of a video picture, each frame having 2621/2 lines in an "odd" field interleaved with 2621/2 lines in an "even" field. Between the end of an even field and the beginning of an odd field there is a corresponding vertical blanking interval of 20 lines duration, and between the end of each odd field another similar but not identical vertical blanking interval of 20 lines duration. There remain 485 horizontal lines of video of each frame which are not blanked out during the vertical intervals, though some of these lines (i.e., at the beginning and end of the frame) are not customarily displayed on a standard TV receiver. This picture format is well known and need not be described further.
Each of the horizontal lines of a video picture comprises a first or horizontal blanking interval (HBI) during which a horizontal line sync pulse and a color burst signal are provided. The remainder of the line is an active video (AV) portion during which color picture information is transmitted. The horizontal sync pulse serves to synchronize a horizontal sweep circuit in a TV receiver (as is well known) and the color burst signal provides synchronizing of the luminance and chromanace circuits of the receiver (also well known). Ordinarily during the HBI, the line video is blanked out and no indication of the presence of the synchronizing signals is evident in the picture displayed on the screen of the TV receiver.
Scrambling of a TV picture can be accomplished very effectively by suppressing the horizontal sync signals thus rendering a TV receiver unable to display a proper picture. In the absence of the necessary timing and sequencing of the horizontal lines of the fields and frame of a TV picture provided by the synchronizing signals, the picture becomes a visual jumble of bits and pieces. To view the picture properly it is necessary to restore the horizontal sync pulses to standard levels.
Circuits which suppress sync signals in a TV picture in order to scramble it are well known in the art. Broadcast equipment embodying such a circuit is located at a central office of a cable TV operator, for example, and is used to scramble restricted TV channels before they are transmitted to subscriber locations. Securely coded control signals generated as part of scrambling are also sent to subscriber locations from the central office. These coded signals enable only the receivers of those subscribers who pay to receive a restricted channel or channels to descramble them for proper viewing. There may be thousands of subscribers in a given cable TV system and therefore it is economically important to the cable system operator that only authorized subscribers have access to the restricted channels and be able to view properly the scrambled pictures.
There are various arrangements previously used to recover suppressed sync signals in a scrambled TV picture. Several of these arrangements will be described in greater detail hereinafter in order to better show the advantages in performance, fidelity and cost obtained by the present invention. While relatively little equipment is needed at a central office of a cable TV system in order to scramble pictures being broadcast, there are needed thousands of receivers, at least one for each subscriber location, able to descramble the pictures. The cable TV operator can afford to spend a substantial sum of money for the equipment at the central office since relatively speaking only a small amount is required. Hence for that equipment there need be no tradeoff in performance and fidelity versus cost. The scrambling equipment can be as good as it needs to be and cost is a secondary consideration. On the other hand each subscriber descrambling receiver, since there may be thousands of them, must be produceable at reasonably low cost (e.g., a hundred dollars or less). Cost factors made it usual, prior to the present invention, to accept certain limitations in performance and fidelity in operation of descrambling equipment to achieve a cost objective.
In recent years, the fidelity and technical quality of commercial TV receivers, of video pictures themselves, and of the distribution networks of cable TV systems have significantly improved. What was thought to be "good" in picture reproduction a few years ago is now seen to have problems of noise, of distortion, and of similar artifacts relating to scrambling and descrambling. The human eye is very sensitive to imperfections in the images it sees. For example, measured against a one volt peak to peak video signal of an image, distortions and other artifacts in the signal of only a few millivolts may be objectionable to a viewer of the image as reproduced on the screen of a high quality TV receiver. Because of cost limitations, previous descrambling arrangements often produce low level, though visually evident artifacts in TV pictures they descramble. It is therefore desirable to provide a descrambling method and apparatus which are economical to employ and which reproduce video images with virtually the same fidelity as the original not-scrambled images. The present invention provides such method and apparatus.